Recently, studies and developments on light emitting diodes (hereinafter referred to as “LEDs”) have rapidly advanced, and various types of LEDs have been developed and commercialized to be used in a wide range of fields, while the use of LEDs in the field of lighting is becoming increasingly popular. In the field of lighting, LEDs are used not only as a substitute for fluorescent lights and light bulbs but also for, for example, backlights for liquid crystal display panels and the like.
For example, the backlights for liquid crystal display panels are roughly classified into an edge-light type in which a light source such as a cathode ray tube or an LED is disposed on one side portion of a light guide plate to illuminate a plane part of the light guide plate, and a direct type in which light sources of a similar kind and a diffusing plate are disposed facing each other with a predetermined distance therebetween, and these light sources directly radiate light onto the diffusing plate to illuminate a diffusing plate surface. The edge-light type backlight requires an expensive light guide plate. Thus, to obtain a large light emitting surface, a large light guide plate is required which leads to a heavier weight and a higher cost. Furthermore, when such a large light guide plate is used, the light path between the light source and a light emitting surface is long and thus, light is largely attenuated. Therefore, a problem exists in that uniform illumination is difficult to obtain.
By contrast, the direct type backlight, which requires no light guide plate, has a problem in that when a point light, source, e.g., an LED, is used as the light source, uniform surface light is difficult to obtain based on the light from the LED. Thus, direct type backlights, with each using an LED, designed to solve the problem are proposed (see, for example, Patent Documents 1 to 3 listed below).
FIG. 25 illustrates the direct-type backlight using the LED described in Patent Document 1 listed below. FIG. 25A is a perspective view of a backlight portion in which an inner portion can be seen through. FIG. 25B is a partial cross-sectional view of FIG. 25A.
A backlight box described in Patent Document 1 listed below is disposed on a rear surface of an unillustrated color liquid crystal display panel and illuminates the liquid crystal display panel from the rear surface. The backlight box includes a box portion 20 provided with an opening through which light from an LED unit described later is emitted outside, and a transmitting/diffusing plate 26 provided over an opening 20a of the box portion 20 in the box portion 20, red LEDs 21R, green LEDs 21G, and blue LEDs 21B are used as light sources. The LEDs are linearly arranged in a predetermined order to form the LED unit.
A casing 23 of the box portion 20 includes inner surfaces including a reflective surface 24. In the casing 23, a transmitting/reflecting plate 25 in parallel with the opening portion 20a is provided at an intermediate portion in such a manner as to cover the LED units. The transmitting/reflecting plate 25 has a function of partly transmitting incident light and partly reflecting the incident light. In the backlight box, a part of the light of each color emitted from the LED unit does not directly transmit through the transmitting/reflecting plate 25. Instead, a part of light transmits through the transmitting/reflecting plate 25 after being subjected to internal reflection for a plurality of times between the reflective surface 24 and the transmitting/reflecting plate 25 in a space closed by the transmitting/reflecting plate 25. As a result, the radiation area of light incident on a transmitting/diffusing plate 26 is increased and desired white light can be obtained with an unevenness in color and brightness reduced. Furthermore, the box casing has a height (thickness) reduced to 2 cm to 5 cm from 8 cm to 10 cm in a conventional technique.
Patent Document 2 listed below describes a direct type backlight in which partition walls having a height of 5 mm to 25 mm disposed among a plurality of LEDs are provided to a configuration similar to that of the backlight described in Patent Document 1 listed below to reduce loss of light. The partition walls are not tall enough to reach the diffusing plate.
Patent Document 3 listed below describes surface lighting light source device including: a box-shaped casing having an upper portion opened and being provided with an LED on a bottom plate; and a light transmitting/reflecting plate covering the opening of the casing and having a central reflecting section at a portion directly above the LED and an outer reflecting section provided at the periphery of the central reflecting section. It should be noted that Patent Document 3 is a publication of a patent application related to the applicant of the present application.
FIG. 26 illustrates the surface lighting light source device described in Patent Document 3 listed below. FIG. 26A is a schematic perspective view. FIG. 26B is a cross-sectional view taken along the line XXVIB-XXVIB in FIG. 26A.
This surface lighting light source device 30 includes: an LED 33; a housing 31 including a square bottom plate 31a in which the LED is disposed at a center portion and side walls 31b to 31e each standing to a predetermined height from the periphery of the bottom plate, having an upper portion opened, and having an inner wall surface formed of a reflective surface; and a light transmitting/reflecting plate 32 that covers an opening 31f of the housing 31. The light transmitting/reflecting plate 32 is divided into a central reflecting section 32a at a portion directly above the LED 33 and an outer reflecting section 32b on the outer periphery of the central reflecting section, and is formed of a material having predetermined reflectance and transmittance.
The central reflecting section 32a is a circular region having a predetermined radius (diameter φ1) from a point where the light transmitting/reflecting plate 32 intersects with a light axis Ax of the LED 33. The surface lighting light source device 30 is assembled by: disposing the LED 33 substantially at the center of the bottom plate 31a of the housing 31; covering the opening 31f of the housing with the light transmitting/reflecting plate 32; and fixing the light transmitting/reflecting plate 32 to the periphery of the opening. As illustrated in FIG. 26B, the height of the thus assembled surface lighting light source device 30 is equal to the protruding height H1 of the LED 33 from the surface of the bottom plate 31a. 
The dimensions of the surface lighting light source device 30 is as follows with the length of one side of the bottom plate 31a of the housing represented by L1, the height of the side plates 31b to 31e of the housing represented by H1, the length of one side of the light transmitting/reflecting plate 32 represented by L1, the thickness of the light transmitting/reflecting plate 32 represented by D1, the diameter of the central reflecting section 33a represented by φ1, the height of the LED 33 represented by h1, and the height from the LED 33 to the central reflecting section 32a represented by h2. Specifically, L1 and of the housing 31 is 100 mm and H1 is 15 mm. L1 of the light transmitting/reflecting plate is 100 mm and the thickness D1 is 1.0 mm. The diameter φ1 is 10 mm, h1 is 1 mm, and h2, is 13 mm.
The thickness of the bottom plate 31a of the housing 31 is 1.0 mm. H1 is the sum of h1 and h2. An area S of the light, transmitting/reflecting plate 32 and an area ΔS1 of the central reflecting section 32a are calculated as follows on the basis of the dimensions. Specifically, S is L1×L1, i.e., 100 mm×100 mm, whereas ΔS1 is π×(½×φ1)2, i.e., π×52 (mm2). An area ratio ΔS1/S1 of the area ΔS1 of the central reflecting section 32a to the area S of the light transmitting/reflecting plate 32 and is 0.78%.